Incandescent lamp having good color rendering properties at a high color temperature

Abstract

Four different optical interference films are formed on the outer surface of an incandescent lamp to achieve good color rendering properties at a high color temperature. A first film formed on the lamp includes high and low refractive index layers alternately stacked so that the total number of layers is n (where n is odd number greater than 5) and the optical thickness thereof is 0.12˜0.17 μm. A second film formed on the first film has the low refractive index layer and the optical thickness of 0.132˜0.2125 μm, which is calculated by the equation (K+1)×d/2, wherein K is 1.2˜1.5 and d is the optical thickness of each refractive index layer of the first film. A third film formed on the second film has at least a high refractive index layer. The total number of refractive index layers of the third film is an odd number of 1˜(n-4) and the optical thickness of each refractive index layer thereof is 0.144˜0.255 μm, which is calculated by the expression (K×d). A fourth film includes the low refractive index layer having the optical thickness of 0.0720˜0.1275 μm, which is calculated by the expression (K ×d/2).

What is claimed as new and desired to be secured by Letters Patent of the United States is: 
     
       1. An incandescent lamp comprising: a transparent bulb;   a filament supported in the bulb;   a first optical interference film formed on one of inner and outer surfaces of the bulb, the first optical interference film including a first refractive index layer having a prescribed refractive index and a second refractive index layer having a predetermined refractive index lower than that of the first refractive index layer alternately stacked one on another, the total number of first and second refractive index layers being n (where n is an odd number greater than five), the optical thickness d of each refractive index layer being 0.12˜0.17 μm;   a second optical interference film including another second refractive index layer, formed on the first optical interference film, the optical thickness of the second refractive index layer of the second optical interference film being indicated by the expression (K+1)×d/2 (where K is 1.2˜1.5);   a third optical interference film formed on the second optical interference film, the third optical interference film including at least the first refractive index layer, the total number of first and second refractive index layers being an odd number of 1˜(n-4), the optical thickness of each refractive index layer of the third optical interference film being indicated by the expression K×d; and   a fourth optical interference film including another second refractive index layer, formed on the third optical interference film, the optical thickness of the second refractive index layer of the fourth optical interference film being indicated by the expression K×d/2.   
     
     
       2. A lamp according to claim 1 further including a third refractive index layer sandwitched between the first refractive index layer of the first optical interference film and the surface of the bulb, the third refractive index layer having a target refractive index between those of the first and second refractive index layers and a thickness of d/2. 
     
     
       3. A lamp according to claim 2, wherein the prescribed refractive index of the first refractive index layer is greater than 2.0. 
     
     
       4. A lamp according to claim 3, wherein the predetermined refractive index of the second refractive index layer is smaller than 1.6. 
     
     
       5. A lamp according to claim 4, wherein the target refractive index of the third refractive index layer is 1.7˜1.9.